1. Technical Field of the Invention
The present invention relates to a method and apparatus for reforming a hydrocarbon-based fuel, alcohol, etc. into a fuel gas containing hydrogen, for industries which use high-purity hydrogen as a fuel, such as for fuel cells.
2. Prior Art
When electric power is generated using fuel cells, hydrogen is supplied to the fuel cells; a fuel gas containing hydrogen is produced from a raw material consisting of hydrocarbon based fuels, e.g. butane or propane, or alcohol based fuel such as methanol; the raw material is reformed in a reforming vessel containing a catalyst, in which a mixture of the fuel gas, steam and air is reformed.
The reforming reaction proceeds at a rather high-temperature and heat is absorbed during the reaction, so when a conventional reforming device is used, the mixed gas is heated sufficiently in a preheater, and using the heat retained in the gas, the temperature of the catalyst is increased, or is otherwise heated by an external means, so as to expedite the reforming reaction.
Recently, a self-heating system is currently used for a reforming device. In the self-heating system, a mixed gas and reforming catalyst are heated by the oxidation of a part of the mixed gas and the gas is reformed by the heat.
If a gas mixture is supplied to one end of a reforming device filled with a partial oxidation catalyst and a reforming catalyst, and if the reformed gas is discharged from the other end after the gas mixture has made contact with the partial oxidation catalyst and the reforming catalyst, then only the upstream portion of the reforming catalyst near the partial oxidation catalyst is over-heated, and the temperature of the downstream portion of the reforming catalyst, located further away from the partial oxidation catalyst increases after a time delay. As a result, the temperature distribution of the reforming catalyst is uneven, therefore, a fairly long time is required before the temperature of the entire reforming catalyst has been increased, so the reforming device cannot be started up quickly.
In addition, because part of the reforming catalyst is over-heated due to the uneven temperatures distribution, deterioration of the catalyst, such as sintering occurs.
Recently, a new reforming device has been developed and is in practical use; the partial oxidation catalyst and the reforming catalyst are installed in multiple layers, so as to distribute the temperature increase of the reforming catalyst more evenly. This type of reforming device is typically classified into the series type shown in FIG. 1, and the parallel type in FIG. 2.
In the series-type reforming device, the reforming room is arranged in multiple stages (3 stages in FIG. 1) and each stage has a partial oxidation catalyst on the upstream side and a reforming catalyst downstream, and a gas mixture containing a fuel vapor such as methanol, steam and a small amount of air is introduced at one end of the device, and the reformed gas is discharged from the other end. To expedite the partial oxidation reaction of the gas mixture, additional air is fed into the second and third reforming rooms. In the series-type reforming device, the temperatures of the reforming catalysts in each stage are increased automatically by the heat of the partial oxidation reaction, and the length of the passage in which the gas mixture contacts the reforming catalyst can be made long, so the advantage of a high reforming rate can be expected.
Conversely in the parallel-type reforming device, partial oxidation catalysts and reforming catalysts are arranged in a number of stages (3 stages in FIG. 2), in the same way as with the series-type device, and each stage is separated from the others, and a gas mixture containing a fuel vapor such as methanol, steam and a small amount of air is supplied to each stage, and a reformed gas is discharged from each stage. Also with this parallel-type device, the temperatures of the reforming catalysts in each stage can be increased evenly using internally generated heat, and because only the gas mixture is distributed to each stage of the reforming device, the construction can be simplified which is an advantage. If part of the reforming catalyst etc. deteriorates accidentally, each stage can be quite easily replaced individually, which is also an advantage.
However, the aforementioned series- and parallel-type reforming devices are accompanied with the following problems.
(1) With the series-type reforming device, air must be supplied to the reforming rooms at the second and subsequent stages from an external source, so the air piping is complicated and requires a dedicated space. The air supplied from outside must be mixed completely with the gas mixture in the small space between adjacent reforming rooms and then fed to the reforming rooms, but this space is normally small, so the mixing often becomes incomplete. As a consequence, inappropriate reactions may sometimes take place, for example, irregularities may occur in the partial oxidation or reforming reactions.
(2) With the parallel-type reforming device, on the contrary, since the fuel mixture such as methanol, steam and air is mixed completely beforehand and then fed to each reforming room, the problems mentioned above for the series-type reforming device do not occur. However, as the length of the passage in which the gas mixture contacts the reforming catalyst is short, the necessary reforming rate may not be obtained when the distribution of reforming catalysts or the distribution of carrier materials are not maintained evenly.
When a reforming device is used for fuel cells in an electric automobile etc., the motor must be started quickly by generating electric power by supplying high-purity hydrogen into the stack of fuel cells as quickly as possible. The device must also be as compact as possible.
However, with a conventional self-heating system of series- or parallel-type reforming devices, compactness of the device is inconsistent with a high reforming rate as described above.
The hydrogen, required to generate electric power in a fuel cell, is produced by a reforming reaction using a raw material consisting of either a hydrocarbon based fuel, such as butane and propane, or an alcohol based fuel, such as methanol. However, because the hydrogen-rich reformed gas produced by the reforming reaction contains a large amount of carbon monoxide (CO) as an impurity, it should be removed before supplying it to a fuel cell that requires high-purity hydrogen. This is because if CO is fed into the fuel electrode of the fuel cell, it is adsorbed by the catalyst in the electrode, poisons the catalyst, decrease the reaction at the electrode, and lowers the electricity-generating performance.
Under these circumstances, the reforming device is normally provided with a CO removal unit filled with a CO removing catalyst, where a selective CO oxidation reaction (CO+xc2xdO2xe2x86x92CO2) or, if required, a CO shifting reaction (CO+H2Oxe2x86x92CO2+H2) occurs, thus the concentration of carbon monoxide is reduced, in this additional mechanism.
With a reforming device that produces hydrogen-rich reformed gas from a hydrocarbon-based fuel or an alcohol fuel, the reforming reaction proceeds endothermically, so heat must be supplied to the reforming unit. In addition, it is also important to supply heat to increase the rate of the reforming reaction. Therefore, in many cases, fuel gas, water and air are heated by an external heat source to a temperature appropriate for the reforming reaction, to produce a high-temperature vapor which is then fed to the reforming unit, or the gas mixture is heated up to such a temperature in the reforming unit where the reforming reaction takes place.
On the other hand, a CO removal unit containing a catalyst mainly intended to decrease the concentration of CO contained in the reformed gas produced in the reforming unit. the selective CO oxidation reaction begins at about 100 to 200xc2x0 C. and a CO shift reaction occurs at about 200 to 300xc2x0 C. In addition these reactions are exothermic, the temperature of the CO removal catalyst should be prevented from increasing in order to obtain a high CO removal rate. For this reason a conventional reforming device of the reforming unit must be designed to be seperate from the CO removal unit, or if an integrated design is used, a thermal insulation material is required to prevent the heat transfer from the reforming unit to the CO removing unit, and a method of cooling the CO removal unit should be used.
Furthermore, carbon monoxide created in the reforming reaction poisons the electrode catalyst in the fuel cell as described above, and interferes with the reaction of the electrode, so it should be removed from the reformed gas by a CO removal reaction. However, since the CO removal reaction is exothermic, if heat is transmitted from the reforming unit to the carbon monoxide removal portion (CO removal unit), the CO removal reaction does not proceed.
Consequently, in an integrated reforming device composed of a reforming unit and a CO removal unit, the heat transfer from the reforming unit to the CO removal unit must be decreased and the loss of heat from the reforming unit at high operating temperatures must be prevented.
Conventionally, the reforming catalyst is installed in a single cylindrical or square vessel, therefore when the device generates a large output, the sectional area of the passages in the catalyst vessel is also large, often resulting in an irregular distribution of fuel gas flow in the catalyst vessel, and a satisfactory reforming reaction is often not achieved.
When the reforming unit is constructed with the reforming catalyst installed in a single catalyst vessel, if even part of the catalyst deteriorates as a result of operating with an unbalanced flow of the gas mixture, the whole reforming unit must be replaced.
The present invention aims at solving the aforementioned various problems. The first object of the present invention is to offer a reforming method and a reforming apparatus, in which the temperature of the reforming catalyst can be increased, evenly and rapidly at the time of starting, a reformed gas with a high degree of reforming can be produced, and the apparatus is compact and can be easily maintained.
The second object of the present invention is to provide a small reforming apparatus that can produce high-purity hydrogen gas by (1) increasing the temperature of the reforming catalyst, while preventing heat losses caused by heat transfer from the reforming catalyst to the outside, (2) adjusting the cross section of the reforming tubes to give an appropriate area taking into account the number of reforming tubes and the output, thereby making the gas mixture flow evenly through the reforming catalyst, and more preferably (3) by improving the CO removal reaction by suppressing the heat transfer from the reforming unit to the CO removal unit.
To achieve the first object of the present invention, two or more reforming rooms (6) are connected in series; a gas mixture (2) of fuel, water and air is supplied to one end of each unit, and a reformed gas containing hydrogen is discharged from the other end; a first catalyst (8a) that catalyzes the partial oxidation of the fuel in an oxygen environment is installed on the upstream side of each of the aforementioned reforming rooms; a second catalyst (8b) that catalyzes the reforming reaction is installed on the downstream side thereof; the above-mentioned gas mixture is supplied directly to one end of each reforming room, and the reformed gas is discharged from the end of the reforming room furthest downstream.
According to the aforementioned reforming method of the present invention, the second catalyst in each reforming room can be evenly and quickly heated up by the internal heating produced by the above-mentioned self-heating effect in each reforming room, thereby reformed gas containing high-purity hydrogen gas can be produced immediately after starting up. In addition, because the length of the passage in which the gas mixture contacts the second catalyst can be made long, the degree of reforming can also be improved.
An identical catalyst that can accelerate both the partial oxidation reaction and the reforming reaction may also be used for the aforementioned first catalyst (8a) and second catalyst (8b).
In a self-heating system currently used in a reforming device, different catalysts are normally used to accelerate the oxidation reaction and the reforming reaction and these are installed on the upstream and downstream sides respectively. However, some catalysts can expedite both the partial oxidation and reforming reactions. When such a catalyst is incorporated, the reforming room is completely filled with the catalyst and the temperature of the catalyst is increased by the self-heating effect, thus the reforming reaction can be initiated very quickly from the start of operation.
The present invention also offers a reforming method using a reforming tube (10) comprised of two or more of the above-mentioned reforming rooms (6) connected in series and a reformer housing (12) that houses the aforementioned reforming tube, wherein a high-temperature heating gas (16) is introduced into the space (14) formed between the reforming tube and the reformer housing, and after the above-mentioned first catalyst (8a) and the second catalyst (8b) have been heated up from outside the reforming room, the gas mixture (2) is supplied into each reforming room and reformed.
The present invention also offers a reforming method with the novel characteristics that a high-temperature heating gas (16) is supplied directly to one end of each of the aforementioned reforming rooms (6), and is discharged from the other end of the most downstream reforming room, and after the above-mentioned first catalyst (8a) and second catalyst (8b) are heated up from the inside of the reforming room, the gas mixture (2) is fed to each reforming room where it is reformed.
To efficiently reform a gas mixture in a reformer, it is considered necessary to heat the reforming catalyst sufficiently, beforehand. According to the above-mentioned reforming method, the first and second catalysts are heated up evenly and satisfactorily in advance from outside and/or inside using a high-temperature heating gas that has been heated using an external combustor etc., and then the supply of heating gas is stopped, and the gas mixture is fed in, therefore, the reforming reaction can take place efficiently immediately after the gas mixture is supplied. In other words, the reforming reaction can be initiated quickly after start-up, and in addition, the cost of the fuels is also saved.
The present invention also provides a reformer equipped with a mixed gas feeding tube (18) that supplies the gas mixture (2) of fuel, water and air and a reforming tube (10) that converts the above-mentioned mixed gas to a reformed gas (4) containing hydrogen, in which the aforementioned reforming tube is comprised of two or more reforming rooms (6) in series, where the gas mixture (2) is fed in to one end thereof and the reformed gas (4) containing hydrogen is discharged from the other end thereof; each of the aforementioned reforming rooms is filled with a first catalyst (8a) for partial oxidation in an oxygen-rich environment on the upstream side and a second catalyst (8b) for reforming downstream, and the above-mentioned mixed gas feed tube is provided with a means of feeding gas (20) that supplies the gas mixture directly to each reforming room.
The reforming rooms are connected in series, and the gas mixture that has been thoroughly premixed is supplied directly to each reforming room, thereby the second catalyst can be heated up by the self-heating effect, at an early stage in each reforming room. In addition, because the gas mixture supplied to the upstream reforming room also passes through the downstream reforming rooms and is discharged from the other end of the most downstream reforming room, the length of the passage in which the gas contacts the second catalysts is long, so the reforming rate can be improved. Compared to a conventional series-type reforming tube, no external piping needs to be introduced, therefore, the construction is simplified and the equipment can be made compact.
In addition, modular reforming tubes can be used, and the number of reforming tubes can be increased or decreased depending on the output required for the reformer. Also, since the gas mixture can be distributed evenly to each unit, the gas mixture that flows through the catalyst can be prevented from being unevenly distributed across the sectional area, so the reforming reaction can be accelerated. In addition, because the reforming tubes of each unit can be replaced, the apparatus can be easily maintained.
Here, the aforementioned means of feeding gas (20) is an outer cylinder (24) that covers at least part of the downstream end and side surface of the aforementioned reforming tube (10), and the circumferential gap (22) between the reforming tube and the cylinder forms a passage for the mixed gas (2); on the side surface of the abovementioned reforming tube, inlet ports (26) are provided to feed the gas mixture to each reforming room from the abovementioned gap; each of the aforementioned inlet ports is provided with flow control mechanisms (28a, 28b) or flow regulate means (32a, 32b) for adjusting the flow of the gas mixture supplied to each reforming room. This construction is also the preferred method of supplying the gas mixture to each reforming room.
The outer cylinder is arranged so that it covers the side surface of the reforming tube, and the gap between the outer cylinder and the reforming tube is used as a flow passage for the gas mixture, thereby piping is no longer needed to supply the gas mixture to each reforming room, so the reformer can be made simple and compact. This outer cylinder can also suppress heat transfer from the reforming room to outside.
The reason that the inlet ports disposed on the reforming tube are provided with flow control mechanisms or flow regulate means is that if simple inlet ports are constructed on the reforming tube to supply the gas mixture, the gas mixture cannot be supplied to each reforming room with the appropriate distribution. More explicitly, because the supplied gas mixture tends to flow into a passage with a low pressure drop, therefore if only inlet ports are provided, most of the gas mixture will flow into the most downstream reforming room. A variable mechanism etc. disposed at each inlet port provides an appropriate pressure drop (load), so that the gas mixture distributes in each reforming room in an optimal manner.
The aforementioned means of feeding gas (20) are composed of a penetration tube (34) with the structure of a hollow tube that makes the above-mentioned gas mixture (2) flow through the inside of at least one reforming room, from one downstream end of the aforementioned reforming tube (10); the above-mentioned penetration tube is provided with inlet ports (36a, 36b) that supply the gas mixture to each reforming room; and at the above-mentioned inlet ports, flow control mechanisms (28a, 28b) or flow regulate means (32a, 32b) are provided to adjust the flow of the gas mixture introduced into each reforming room, using the aforementioned means, therefore the gas mixture can be fed appropriately to each reforming room.
The gas mixture can also be supplied to each reforming room from the inside of the reforming tube using a penetration tube in place of the above-mentioned outer cylinder. In this case, flow control mechanisms or flow regulate means are also arranged at the inlet ports for the same reason as described above. Here, the flow control mechanisms and flow regulate means may be composed of flow control valves and orifices, respectively.
Other preferable configurations according to the present invention include the provision of a reformer housing (12) that houses the aforementioned reforming tube (10) and an initial heating gas tube (38a) that introduces high-temperature heating gas (16) into the space (14) formed between the above-mentioned reformer housing and the aforementioned reforming tube; then or after the reforming room has been heated up from the outside, a second heating gas tube (38b), connected to the aforementioned mixed gas feed tube (18) introduces high-temperature heating gas (16) from the outside, and after the reforming room has been heated up from the inside, the gas mixture is supplied.
The high-temperature heating gas, after being heated up in a combustor etc., is introduced into the space between the reformer housing and the reforming tube, and preferably it is directed towards the reforming tube, thereby the reforming tube and the catalyst are heated up from the outside, or by supplying the heating gas to each reforming room through the mixed gas feed tube, the catalyst etc. can be heated up satisfactorily from the inside, and then the introduction of the heating gas is stopped, and the gas mixture is introduced. According to this method, the reforming reaction can be implemented quickly and efficiently from the beginning.
To achieve the aforementioned second object, the present invention provides a reforming apparatus that converts a mixed gas (102) comprised of fuel gas, steam and air, into hydrogen; the above-mentioned reforming apparatus is composed of a heating unit (104) that vaporizes and heats the aforementioned gas mixture, a distribution tube (108) that evenly distributes the heated gas mixture to a plurality of branch ports (106) at one end thereof, a reforming unit (114) filled with a reforming catalyst (112) to catalyze a reforming reaction in the aforementioned gas mixture, a manifold (116) in which the above-mentioned distribution tube is disposed, a CO removal unit (124) fully filled with a CO removing catalyst (122) that catalyzes the CO removal reaction of the gas (118) reformed in the aforementioned reforming unit, and a casing (126) that houses the above-mentioned reforming unit, the aforementioned manifold and the above-mentioned CO removal unit; the aforementioned reforming unit is configured with a reforming room (132) and a feedback mechanism (134), in which the reforming room is composed of a reforming tube (130) one end of which is connected to the aforementioned branch port and reformed gas is discharged from the other end thereof, or two or more such reforming tubes arranged in parallel, and the feedback mechanism allows the above-mentioned reformed gas to flow through the outer periphery of the aforementioned reforming tube and sends the gas to the above-mentioned manifold.
The gas mixture (102), vaporized and heated in the heating unit (104), is distributed through the distribution tube (108) and is supplied to one reforming tube (130) or a plurality of tubes (130), and undergoes a reforming reaction in the reforming tube or tubes. Here, an orifice or a sintered panel or the like is provided at the inlet of the distribution tube, thus the gas mixture is distributed to the reforming tube or tubes; and the cross section of the reforming tube is adjusted according to the relationship between the number of reforming tubes and the output, to give an optimum area, that is, when a small amount of the reformed gas is demanded, the number of reforming tubes is reduced, and a reforming tube with a slightly smaller sectional area is used; when a large amount of reformed gas is required, the number of reforming tubes is increased and also a reforming tube with a slightly larger cross section is used, thereby the gas mixture is distributed evenly across the cross section and along the length of each reforming tube, and in this way, the gas mixture can be diffused uniformly into the interior of each reforming tube. As a result, the gas mixture and the reforming catalyst can be made to contact each other efficiently, and the reforming reaction can be expedited.
In addition, by sending the high-temperature reformed gas to the manifold (116) through the outer periphery of the reforming tube, heat losses from the reforming tube to the outside can be decreased.
Here, the aforementioned CO removal unit (124) can preferably communicate with the above-mentioned manifold (116), and be positioned opposite the aforementioned reforming unit (114).
According to the reforming apparatus of the present invention, because the reforming unit (114) wherein a reaction takes place at a rather high temperature can be connected freely to the CO removal unit (124) in which another reaction occurs at a temperature lower than the above temperature, heat transmission from the reforming unit to the CO removal unit can be prevented by, for example, positioning the manifold between them, so even if the reforming unit and the CO removal unit are formed as an integral unit, the reforming apparatus can be made smaller in size.
In the above, the aforementioned feedback mechanism (134) may also preferably send the above-mentioned reformed gas (118) to the aforementioned manifold, through the space between the aforementioned adjacent reforming tubes (130) or through a reformed gas passage (136) consisting of a longitudinal gap parallel to the axis of the reforming tube, formed between the above-mentioned reforming tube and the aforementioned casing (126).
The gap created between adjacent reforming tubes (130) or between the aforementioned reforming tube and the above-mentioned casing (126) can be used as a passage (136) for the reformed gas, and by sending the high-temperature reformed gas (118) to the manifold (116) along the outer periphery of the reforming tube, the high-temperature reformed gas can completely fill the space around the outer periphery of the reforming tube, thus efficiently suppressing heat transfer from the reforming tube to the outside, and special piping etc. is no longer needed to send the reformed gas to the manifold, therefore, the construction of the apparatus can be simplified.
In addition, the aforementioned reforming tube (130) can preferably be removable and replaceable.
Because the reforming tube (130) filled with the reforming catalyst (12) is structured as a modular unit, each reforming tube can be inspected and replaced, so the apparatus can be maintained more easily than in the prior art.
Moreover, a fuel trap unit (138) that removes fuel gas from the reformed gas (118) can be disposed between the aforementioned manifold (116) and the above-mentioned CO removal unit (124).
The fuel trap unit (138) installed between the manifold (116) and the CO removal unit (124), can prevent fuel gas that was unreformed in the reforming unit after entering the CO removal unit, and adhering to the CO removal catalyst, resulting in interference with the CO selective oxidation reaction or the CO shift reaction, thus, CO can be removed efficiently, and at the same time heat produced in the reforming unit (114) can also be prevented from being transmitted to the CO removal unit and the reformed gas (118) can be cooled in the fuel trap unit.
It is also preferred that a feed tube (142) is provided that supplies oxygen, air or steam to the reformed gas (118) as it is being sent from the aforementioned manifold (116) to the above-mentioned CO removal unit (124).
As oxygen (air) or steam is supplied to the reformed gas (118) as the mixture is being sent into the CO removal unit, an appropriate amount of oxygen and steam can be provided to satisfy the above-mentioned selective CO oxidation reaction (CO+0.5O2xe2x86x92CO2) or the CO shift reaction (CO+H2Oxe2x86x92CO2+H2), and at the same time, by cooling the reformed gas, the temperature of the CO removal unit can be prevented from increasing excessively, and so the CO removal reaction can proceed more rapidly.
Here, the aforementioned CO removal unit (124) is composed of one partition or two or more partitions; on the upstream side of each partition, feed tubes (142a, 142b) can be constructed to supply oxygen, air or steam.
For example, the CO removal unit can be divided into two partitions; a steam feed tube is installed in front of the upstream partition filled with a catalyst appropriate for the CO shift reaction, and an oxygen feed tube is provided before the downstream partition charged with a catalyst suitable for the selective CO oxidation reaction, thus CO can be removed efficiently, and a reformed gas (refined gas) with a higher hydrogen purity than in the prior art can be produced.
Other objects and advantages of the present invention are revealed in the following paragraphs referring to the attached drawings.